This invention relates to a structure and method by which lyotropic chromonic liquid crystals are aligned at a surface as one monomolecular layer or as a stack of individual molecular layers, each layer possessing long-range in-plane orientational order.
Liquid crystals are traditionally classified into thermotropic and lyotropic types. Thermotropic materials acquire their mesomorphic (orientationally ordered) state when the material is within a certain temperature range. Lyotropic materials become mesomorphic when dissolved in some solvent (such as water), within an appropriate concentration range.
Alignment of thermotropic liquid crystals is an active area of current research and development. Usually, the alignment technique is based on a special unidirectional treatment of the plates or substrates that bound the liquid crystalline material. An example technique is disclosed in U.S. Pat. No. 5,596,434 entitled xe2x80x9cSelf-Assembled Monolayers For Liquid Crystal Alignment.xe2x80x9d The ""434 patent discloses that the plates are covered with a polymer layer which is mechanically rubbed. The direction of rubbing sets the direction of orientation of the liquid crystal, i.e., the director, at the substrate, as a result of anisotropic molecular interactions at the interface. The phenomenon of orienting action between the anisotropic (rubbed, for example) substrate and the liquid crystalline alignment is called xe2x80x9canchoring.xe2x80x9d Alignment by surface anchoring is a standard means of alignment in liquid crystalline displays. Surfaces are typically treated with a polymer or a surfactant in order to obtain the desired alignment effects. The methods of alignment are well established for thermotropic liquid crystals but are not necessarily applicable to lyotropic liquid crystals because of the differences in the molecular structure between the two classes of liquid crystals.
Lyotropic liquid crystals are more difficult to align in the plane of the substrate than their thermotropic counterparts. The reason is that most lyotropic liquid crystals are based on amphiphilic materials (surfactants) dissolved in water or oil. Amphiphilic molecules have a polar (hydrophilic) head and a non-polar (hydrophobic) aliphatic tail. When surfactant molecules are in contact with a substrate, their amphiphilic nature generally results in a perpendicular orientation of the molecule with respect to the plane of the substrate. Either the polar head or the hydrophobic tail of the molecule is attracted to the polar or non-polar bounding plate, which results in the perpendicular alignment of the molecule with respect to the substrate. Perpendicular alignment means that the preferred orientation is the so-called homeotropic alignment, in which the optical axis is perpendicular to bounding plates. However, it is very difficult, if possible at all, to align the surfactant-based liquid crystal in a planar fashion where the director is in the plane of the solid substrate.
There is a special class of lyotropic liquid crystal, called lyotropic chromonic liquid crystal (LCLC). The LCLC family embraces a range of dyes, drugs, nucleic adds, antibiotics, carcinogens, and anti-cancer agents. The molecular and macrostructure of LCLCs, as seen in FIG. 1B, are markedly different from that of conventional lyotropic liquid crystals based on amphiphilic rod-like molecules with polar heads and hydrophobic alkyl chain tails, also referred to as surfactants which are shown in FIG. 1A. LCLC molecules are believed to be plank-like rather than rod-like, rigid rather than flexible, aromatic rather than aliphatic. According to Lydon, the interaction of the aromatic cores is the main mechanism of molecular face-to-face stacking. Hydrophilic ionic groups at the periphery of the molecules make the material water-soluble. These materials have become a subject of intensive studies lately as it became clear that they can be used as internal polarizing elements in liquid crystal displays. These applications require a uniform alignment of LCLC materials with the director in the plane of the cell (Or slightly tilted). To achieve an in-plane alignment, Ichimura et al. suggested modifying the chemical composition of the LCLC by adding an unspecified non-ionic surfactant (0.4%-1% wt.). A photo-treated polymer is then used to align the mixture of surfactant and LCLC. In the present invention, we achieve alignment of LCLCs without modifying their composition with surfactants. The established art of aligning LCLCs in bulk solutions uses a strong magnetic field applied to the LCLC cell. Unfortunately, this field-induced alignment is only temporary as the degenerate (no fixed direction of molecular orientation) orientation returns within tens of minutes once the magnetic field is removed.
There are also known techniques for layer-by-layer electrostatic deposition of materials that form surface film alignments. Adsorption of charged colloidal particles on a layer-by-layer basis is a technique that was originally developed by Iler in 1965. The technique was expanded to include adsorption of anionic and cationic polyelectrolytes on a charged surface. In addition, this technique is very effective in investigating two-dimensional aggregation of dye monolayers on polyanion subphases. An extensive amount of work on creating stacked layers of proteins, dyes, SiO2 nanoparticles, and charged polysaccharides on polyions has also been done. The basic concept consists of using oppositely charged materials to adsorb one layer onto the other alternately. The layer thickness is determined to be no more than a molecular layer due to the effective screening of ionic charges.
One of the challenges of self-assembly techniques is the control of in-plane orientation of microdomains. As noted above, in bulk samples, uniform alignment is achieved by using liquid crystal materials or by shear of polymer melts, but it is not clear how or even whether these methods can be applied to films of nanometer thickness. Morkved et al. has shown that the local control of orientation could be achieved by using a substrate with patterned electrodes. An in-plane electric field orients a dielectrically anisotropic material, for example, a block copolymer film. The oriented area between the electrodes cannot be much wider than few tens of micrometers since the electric field needed to align the structure increases with the separation distance between the electrodes.
In-plane orientation of a molecular monolayer has never been achieved in the known art using self-assembly of charged species. The idea of employing properties intrinsic to the liquid crystalline phase to create an oriented monolayer in Langmuir-Blodgett (LB) techniques has already been well-established. However with LB techniques, the short-range orientation is normal (perpendicular) to the film plane as the polar groups are either on the top (or the bottom) of the film with the non-polar tails on the opposite side as seen in FIG. 2. In addition, LB films are notoriously difficult to make uniformly over large areas and the molecules used in LB techniques generally exhibit a very low birefringence and little to no light absorption.
It is thus an aspect of the present invention to provide alignment of lyotropic chromonic liquid crystals at surfaces as monolayers.
It is another aspect of the present invention to provide a film on a substrate, wherein the substrate is treated to exhibit a desired polarity.
It is a further aspect of the present invention to provide a structure, as set forth above, wherein the film includes a polyion layer on the substrate, which may or may not be sheared, such that the polyion""s polarity is attracted to the polarity of the substrate.
It is yet another aspect of the present invention to provide a structure, as set forth above, wherein the film includes a lyotropic chromonic liquid crystal layer disposed on the polyion layer, and wherein the lyotropic liquid crystal layer may or may not be sheared.
It is yet another aspect of the present invention to provide a structure, as set forth above, wherein the polarity of the lyotropic chromonic liquid crystal material is attracted to the polyion layer""s polarity.
It is still another aspect of the present invention to provide a structure, as set forth above, in which additional film layers of polyion and lyotropic liquid crystal material may be added.
It is still a further aspect of the present invention to provide a structure, as set forth above, in which each film layer may have its own orientation as a result of shearing in that particular direction.
It is an additional aspect of the present invention to provide a structure, as set forth above, in which the polyion/lyotropic film may be used as an alignment layer for thermotropic liquid crystal materials.
The foregoing and other aspects of the present invention, which shall become apparent as the detailed description proceeds, are achieved by a lyotropic chromonic liquid crystal structure, comprising a substrate and at least one lyotropic chromonic liquid crystal surface film disposed on the substrate, the film comprising a polyion layer disposed on the substrate and a monomolecular lyotropic chromonic liquid crystal layer disposed on the polyion layer.
Still another aspect of the present invention is attained by a method for forming oriented monolayers of lyotropic chromonic liquid crystals, comprising the steps of providing a substrate, disposing a polyion layer on to the substrate, and disposing a monomolecular lyotropic liquid crystal layer on to the polyion layer.
Yet further aspects of the present invention are attained by a method for forming an aligned liquid crystal cell, comprising the steps of providing a pair of substrates, disposing a polyion layer on to each substrate, disposing a lyotropic liquid crystal layer on to each polyion layer, wherein the polyion layer and the liquid crystal layer form a film, positioning the lyotropic liquid crystal layers so that they face one another and provide a gap therebetween, and filling a thermotropic liquid crystal material into the gap.
These and other aspects of the present invention, as well as the advantages thereof over existing prior art forms, which will become apparent from the description to follow, are accomplished by the improvements hereinafter described and claimed.